It is known to use laser-based and/or imager-based readers or scanners in a dual window or bi-optical workstation to electro-optically read indicia, such as bar code symbols, associated with three-dimensional products to be identified and processed, e.g., purchased, at a point-of-transaction workstation provided at a countertop of a checkout stand in supermarkets, warehouse clubs, department stores, and other kinds of retailers. The products are typically slid or moved in various directions by a user across, or presented to a central region of, a generally horizontal window that faces upwardly above the countertop and/or a generally vertical or upright window that rises above the countertop. When at least one laser scan line generated by a laser-based reader sweeps over a symbol and/or when return light from a symbol is captured over a field of view by a solid-state imager of an imager-based reader, the symbol is then processed, decoded and read, thereby identifying the product.
The symbol may be located low or high, or right to left, on the product, or anywhere in between, on any of six sides of the product. The symbol may be oriented in a “picket fence” orientation in which elongated parallel bars of a one-dimensional Universal Product Code (UPC) symbol are vertical, or in a “ladder” orientation in which the UPC symbol bars are horizontal, or at any orientation angle in between. The products may be held by the user at various tilt angles during their movement across, or presentation to, either window. The products may be moved relative to the windows in various directions, for example, from right-to-left, or left-to-right, and/or in-and-out, or out-and-in, and/or high-to low, or low-to-high, or any combination of such directions, or may be positioned either in contact with, or held at a working distance away from, either window during such movement or presentation. All these factors make the symbol location variable and difficult to predict in advance.
As advantageous as workstations with laser-based readers have been in processing transactions, workstations with imager-based readers, also known as imagers or cameras, are thought to offer improved reliability and have the added capability of reading indicia other than UPC symbols, such as two-dimensional or stacked or truncated symbols, as well as the capability of imaging non-symbol targets, such as receipts, driver's licenses, signatures, etc. It was initially thought that an all imager-based workstation would require about ten to twelve, or at least six, imagers in order to provide a full coverage scan zone to enable reliable reading of indicia that could be positioned anywhere on all six sides of a three-dimensional product. However, to bring the cost of the imager-based workstation down to an acceptable level, it is known to reduce the need for the aforementioned six to twelve imagers down to two imagers, or even one imager, by splitting the field of view of at least one imager into a plurality of subfields of view, each additional subfield serving to replace an additional imager. The scan zone extends above the horizontal window and in front of the upright window as close as possible to the countertop, and sufficiently high above the countertop, and as wide as possible across the width of the countertop. The scan zone projects into space away from the windows and grows in volume rapidly in order to cover indicia on products that are positioned not only on the windows, but also many inches therefrom.
Each imager includes an array of image sensors, and typically has an associated illumination assembly to illuminate the indicia with illumination light. The image sensors detect the return illumination light reflected and/or scattered from the indicia. Each imager preferably operates at a frame rate of multiple frames per second, e.g., sixty frames per second, each frame lasting about 16.67 milliseconds. Each subfield of view, also known as a light collection region, is preferably individually illuminated and extends through at least one window over regions of the product. Each imager includes either a global or a rolling shutter to help prevent image blur, especially when the indicia passes through the scan zone at high speed, e.g., on the order of 100 inches per second. Preferably, to reduce power consumption, to prolong operational lifetime, and to reduce bright light annoyance to operators and customers, the illumination light is not emitted at all times, but is emitted only when a proximity sensor detects the presence of a product entering the workstation. Upon product detection, the proximity sensor causes the illumination assembly to be activated.
To insure good reading performance, each imager must be properly exposed, and such aforementioned variable factors as the working distance, orientation, direction, speed and position of the indicia, as well as the light transmissivity of each window, must be taken into account. To achieve such proper exposure, it is known to provide an imager with an internal auto-exposure circuit for measuring the intensity level of the return illumination light in the field of view of the imager, and for adjusting the exposure duration of the imager. As advantageous as such an internal auto-exposure circuit is, it adjusts the exposure duration of the imager in which it is internally integrated, typically only after a few frames have elapsed. This non-negligible time delay creates a sluggishly performing workstation and may, in some cases, even be perceived as defective.